Molecular biology of positive strand RNA viruses

Viruses and global warming paper in Current Biology

Current Biology

Not as unconnected as you might think. The most numerous photosynthetic organisms on earth – the cyanobacteria – are infected by viruses (cyanhophages). Some of these cyanhophages carry components of the photosynthetic machinery and are thought to contribute to host cell photosynthesis. In a recent study on which we collaborated we show that virus-infected cyanobacteria are inhibited in their ability to fix CO2 (in contrast to uninfected cyanobacteria) whereas photosynthetic electron transport is unaltered. The cyanhophages therefore redirect photosynthesis to support phage development.

These results also have implications for our understanding of global warming. The reduction in CO2 fixation in the marine environment, as a consequence of these cyanophage infections, may be as much as 10%. The global warming calculations are based on assumptions of carbon fixation levels being directly linked to photosynthetic activity. We show that that this is incorrect and that CO2 fixation is likely overestimated in marine environments.

Marine picocyanobacteria of the genera Prochlorococcus and Synechococcus are the most numerous photosynthetic organisms on our planet. With a global population size of 3.6 × 1027, they are responsible for approximately 10% of global primary production. Viruses that infect Prochlorococcus and Synechococcus (cyanophages) can be readily isolated from ocean waters and frequently outnumber their cyanobacterial hosts. Ultimately, cyanophage-induced lysis of infected cells results in the release of fixed carbon into the dissolved organic matter pool. What is less well known is the functioning of photosynthesis during the relatively long latent periods of many cyanophages. Remarkably, the genomes of many cyanophage isolates contain genes involved in photosynthetic electron transport (PET) as well as central carbon metabolism, suggesting that cyanophages may play an active role in photosynthesis. However, cyanophage-encoded gene products are hypothesized to maintain or even supplement PET for energy generation while sacrificing wasteful CO2 fixation during infection. Yet this paradigm has not been rigorously tested. Here, we measured the ability of viral-infected Synechococcus cells to fix CO2 as well as maintain PET. We compared two cyanophage isolates that share different complements of PET and central carbon metabolism genes. We demonstrate cyanophage-dependent inhibition of CO2 fixation early in the infection cycle. In contrast, PET is maintained throughout infection. Our data suggest a generalized strategy among marine cyanophages to redirect photosynthesis to support phage development, which has important implications for estimates of global primary production.